This invention relates to a sheave assembly incorporating a ball bearing supporting a rotatable sheave. Further, this invention relates to a pulley block incorporating such a sheave assembly.
Though a pulley block incorporating a sheave assembly of this invention may be used in a wide variety of circumstances, where a rope or other flexible tension member is to be guided through some angle by the pulley or as a part of a tackle, the invention is primarily intended for a pulley block to be used on, high performance sail-boats and racing dinghies in particular, where excellent reliability is demanded, in conjunction with high strength but low weight. Consequently, the invention will hereinafter be described primarily with reference to such intended use, even though it is to be understood that the invention is not to be regarded as limited to that use.
A typical pulley block (often referred to just as a xe2x80x98blockxe2x80x99) employs a sheave (pulley) rotatably mounted on a block pin by means of a ball bearing. The pin is carried within a shell having a pair of side cheeks, a strap (or strop) extending around the shell to permit mounting of the block. The ball bearing typically has two axially-spaced rows of balls which run on inner and outer races provided respectively on the pin and within the bore of the sheave. When both the pin and sheave are of metal, the respective races may be formed directly on the surface of the pin and within the bore of the sheave, rather than by providing a separate ball bearing assembly including inner and outer races together with caged balls.
A modern high performance pulley block intended for use for example on a racing dinghy is manufactured mainly from plastics materials, rather than metals. Thus, the block cheeks, as well as at least some of the sheave, inner race and balls may be made of plastics materials by suitable moulding operations, with the sheave itself providing the outer race. Unfortunately, there is a tendency particularly with dinghy sailors to employ smaller (and so lighter) blocks and to subject those blocks to higher loads. This has led to premature failure of the blocks, the particular component which fails depending upon the precise construction employed. For example, if the inner race and sheave are made of plastics materials and stainless steel balls are employed, the inner race may fail consequent upon by the loads imparted thereto by the balls. Thus, it has proved necessary either to employ a block of a significantly greater size, or a block with metal races and balls, if premature failure is to be avoided but both of these solutions have the disadvantages of increasing the weight of the block as well as introducing higher costs.
In an attempt to address the above problems, the present invention provides a sheave assembly comprising an inner race, a sheave defining an outer race and a plurality of balls arranged between the outer race of the sheave and the inner race thereby rotatably mounting the sheave co-axially on the inner race, wherein the inner race is formed principally of a plastics material but incorporates an annular insert of a relatively hard material, the outer surface of the insert extending substantially parallel to the axis of the inner race and defining an inner track around which the balls run, and the axial width of the insert being significantly less than the diameter of each ball.
It will be appreciated that by employing a sheave assembly of the present invention in a pulley block, the weight of the pulley block will be increased only relatively slightly, as compared to a block having an inner race wholly of a plastics material. However, the relatively high point-contact loads between the balls and an inner race of a plastics material are avoided, so much greater strength, efficiency and durability and consequently also reliability may be expected from a block incorporating a sheave assembly of this invention, as compared to a block having an inner race solely of a plastics material. In addition, the quantity of metal employed in the block is small and so the cost associated with the use of relatively expensive corrosion-resistant metals is minimised.
Most preferably, the annular insert is manufactured from a metal, such as a marine grade of stainless steel, though it could be formed from other metals such as brass, a bronze or an aluminium alloy. Each ball may be of any suitable material having regard to the intended use of the block; for example, the balls may be of stainless steel, tungsten, ceramics or plastics materials.
In a sheave assembly of this invention, each insert has an essentially cylindrical outer surface parallel to the axis of the inner race. Thus, there is essentially a point contact between each ball and the annular insert. Consequently, the insert may have a relatively narrow width, in the axial direction, so long as there is sufficient width on which the balls may run, taking into account manufacturing tolerances, running clearances and an allowance for expected wear, during the life of the block. On the other hand, a narrower insert reduces yet further the overall weight of a complete block. Typically, therefore, the insert will have an axial width not greater than about ⅔ of the diameter of each ball running on the insert, though the width may be reduced yet further, perhaps to no more than 50% of the ball diameter.
Though the sheave assembly of this invention could have a single row of balls supporting the sheave on a single insert, it is highly preferred that a sheave assembly or a block using the sheave assembly has two rows of balls, the sheave defining an outer race having two tracks respectively for the two rows of balls. In this case, the inner race should have two annular inserts spaced axially therealong and defining two inner tracks, respectively for the two rows of balls. Such an inner race may conveniently have three plastics parts each manufactured by a moulding operation, these parts comprising a central section and a pair of end flanges mounted one onto each end of the central section, with the two annular inserts being carried between the central section and each end flange, respectively. In a preferred arrangement, the central section has an axially-projecting boss formed at each end, an insert and an end flange being mounted on each boss, respectively. Each end flange may define a side thrust surface for the balls, whereby attachment of the end flange to the associated boss, on assembling the block, will retain the sheave on the inner race. Conveniently, each end flange is simply an interference friction fit on its boss, though the flange could be secured to the boss by means of an adhesive which chemically bonds together the two components.
Another possibility is to utilise the side cheeks of a block within which the assembly is installed to maintain the inserts in position on the central section, with the side cheeks being profiled to take any side thrusts. In an alternative arrangement, the inner race may be a one-piece plastics material moulding, with the or each annular insert incorporated therein during the moulding operation.
The sheave advantageously is a one-piece plastics material moulding. In view of the greater diameter of the bore of the sheave, as compared to the diameter of the inner race, the outer race for the balls may be formed directly in the sheave, without the need to employ a separate race within the sheave and defining the ball tracks. In the alternative, the sheave could be of other materials, such as an aluminium alloy.
To reduce weight and the cost of the balls, it may be preferred for a cage to be provided for the balls of the bearing, to hold each ball spaced from its neighbouring balls. However, for small-sized blocks ( less than 20 mm diameter sheaves), it is preferred to employ uncaged balls, even though this requires the use of more balls.
The sheave assembly of this invention may be incorporated in a simple pulley block comprising a shell defined by a pair of side cheeks, together with a pin extending therebetween and on which is carried the inner race, between the side cheeks. In the alternative, the inner race itself could form the block pin, and so be directly mounted in the side cheeks of the shell.
If required, a plurality of the sheave assemblies may be mounted between the side cheeks of a block, either on a common pin so as to be co-axial, or on individual pins spaced from each other. In the latter case, the sheaves of the assemblies should be of different diameters, to permit free running of ropes around the sheaves when the block is employed in a tackle.
This invention extends to a pulley block incorporating one or more sheave assemblies of this invention, as described above. In such a pulley block, there may be a hole extending axially through the inner race, in combination with a pulley shell defining a pair of side cheeks and a block pin carried by the side cheeks which block pin extends through the hole of the inner race so as to support the sheave assembly between the side cheeks. As mentioned above, the side cheeks may be profiled to accept side thrust from the balls mounting the sheave on the inner race.